1. Field of the Invention
The present invention relates to an FIR filter to be used in portable terminals or the like in mobile communication systems.
2. Description of the Related Art
Since an FIR (Finete Impulse Response) filter has liner phase characteristics, a transfer function, and stability which cannot be realized by an analog filter, it is utilized for various usages such as communication systems and audio apparatuses. For example, the FIR filter is applied to a filter in a CDMA (Code Division Multiple Access) system and so on, which is one of digital mobile communication systems. In the CDMA system, a transmission rate is variable so an input resolving power enough to express a ratio of change amounts in transmission rate is necessary. Therefore, the resolving power (a bit width) of input data to a filter is required to be increased as a transmission rate increases. A resolving power for obtaining a sufficient distance between signal points is also required for a code-multiplexed transmission of the CDMA system.
Generally, the FIR filter is composed of a shift register, a multiplier, an adder, and so on. These circuits need to be constituted to correspond to the maximum resolving power of input data. As a result, the circuit scale of the filter is enlarged according to a bit width of the input data. The circuit scale of the multiplier is in particular enlarged to a large extent according to an increase in bit width of the input data. However, the above-mentioned input resolving power does not vary all the time. In general, an interval at which the transmission rate changes is sufficiently longer than a time needed for the input data to pass through the shift register.